Following an interesting comment, I’ve decided to check again Ruby on Rails. Very nice piece of work too, but like Symfony, a very dumb policy for migration between versions. If you want to upgrade a project fromRails 2.3 to the new shining Rails 3, you can choose between spending at least one hour and $9 for an online video, or spending $6 for “Almost 120 pages of upgrade information” (!!??), and obviously, an incredible amount of time for boring and useless rewriting.
At least, unlike Symfony, some people keep migration in mind (if you accept to spend time and money for it), anyway if I were a long-time RoR user, I would feel laughed at.

That disease has a name : Planned Obsolescence. But I’m not sure that web developers (and their employers) will still blindly follow a such immature and profit-oriented project policy, applied to Open Source web frameworks.